Hmmmm… I've belly-ached about people blindly following their GPS receivers before.
Now, two independent articles come to hand within two days of each other, via different channels. Both are on problems with GPS. Neither one cites the other (no surprise). Neither one cites any of the other's sources. Yet they come to similar conclusions. Both provide yet more reasons to take GPS data with a salt shaker handy.
David Hambling, in GPS chaos: How a $30 box can jam your life, concentrates on GPS jamming and how it can spoil a lot more than just your car's GPS receiver.
Lewis Page's Chicken Little report: Sat-nav dependency spells DISASTER! covers this and other possible GPS problems from the point of view of a former Royal Navy navigator. It concentrates more on marine navigation and enumerates several alternatives to GPS. It is also aimed at The Register's more technical audience.
Both point out that jamming GPS devices is quite easy, as the GPS signal is very faint. A reasonably robust jammer dropped next to a major airport near a major naval base or shipping harbor could cause considerable disruption.
Page, in his Royal Navy mine sweeping days, used the Mk I eyeball, charts and related tools, and radar, as well as a plethora of electronic aids such as DECCA, a version of LORAN. Your correspondent, in his youthful yachting days on the east coast of North America, used charts, sextant, fathometer, and the Mk I eyeball for everything from harbor piloting to ocean passages. No radar. No LORAN.
I agree with Page's assessment: GPS is great for navigation, but it is no substitute for good training, discrimination on the part of the navigator and other officers, and using your eyeballs. Jam GPS in a major shipping port, and most ships would revert to radar and eyeball. Aircraft would have worse problems, but none are insoluble.
Both articles point out that a major use of GPS is providing precise time signals. Here, too, sole dependence on GPS is potentially messy (but probably not catastrophic). Consider a marine gyrocompass that uses GPS to improve its accuracy: what happens when the GPS signal goes south? The gyrocompass shuts down. Wrong!
One solution is to design systems to use alternatives to GPS when appropriate. Gyrocompasses are very accurate devices without GPS. Why not turn on an indicator that says, "I've lost GPS" and continue on without it? Similarly with mobile phones: if the GPS fails, use triangulation with local towers. Etc.
Another, parallel, solution is simply for the people in charge of these things to be aware that they can fail, and to know what to do when they do. That gets back to training, appropriate skills, and situational awareness. And that's the hard part.